Acid and basic dyes

Acid dyes differ from direct dyes only in that their molecules are smaller, better brilliance can be obtained and they are able to dye protein fibres from an acid solution. Acid dyes show little affinity for cellulose and hence, in the light of what has been said in the previous section, their wet fastness properties are poor.

Acid and basic dyes

The printing method and the need to improve fastness are common to both acid and direct dyes. Both classes are of little importance in textile printing; they are used only for the production of cheap goods. Such goods can also be printed with basic (cationic) dyes, which dye by a completely different mechanism and can be dyed on cellulose only with the help of a mordant. Tannin, a glycoside of gallic acid, and tartar emetic (antimony potassium tartrate) together form a mordant on the cellulose in the form of an insoluble compound, which has not been defined in any detail chemically but which possesses acid groups.

The dye cation can form a water-fast complex salt with these acid groups during a lengthy steaming process. Dyeings and prints produced in this way have a high degree of brilliance, but poor fastness to light. Synthetic products, such as those marketed under the names Katanol ON and Thiotan MS, may be used instead of tannin. These products are in some cases applied together with the dyes from the printing pastes, but then a longer steaming process for fixing is required. This process is no longer very important today.